I also started the game on tactical and while struggling on the start, I have reached level 8 and it appears to be a little easier now, also because I know better the game mechanics.
Here are my advices
(some of them may be obvious):
1) this is a must: always try to use the environment to your advantage. For example use air on water to stun enemies or fire to burn them. It takes some time to understand the relationships between water, air, venom, etc... For example igniting a venom cloud result in smoke which can help your party avoid enemy archers.
2) Recon first and if possible never try to battle with multiple enemies above your level. Use a character with high lore skill to inspect the enemy weaknesses.
3) try to concentrate on one enemy at a time by using spells or scrolls which takes temporarily enemies out of combat.
4) since undead do not like fire, I think it is better to start with a wizard who can do fire damage. If you don't have wizards, use molotov grenades, may be with the "catapult" ability, which increases the range.
5) try to lure away the enemy from his position, may be around an angle so that you can concentrate fire. In an open battle you have little chance of winning on tactical level.
The battle system is quite complex, so take your time to experiment, it's really one of the best part of the game.
Good luck !
